1. Field of the Invention
The present invention relates to a switching power supply device that converts a high DC voltage acquired by rectifying and smoothing a commercial power supply into a low DC voltage required by an apparatus, and an image forming apparatus that includes the power supply device.
2. Description of the Related Art
In recent years, power conservation has been required for various electronic apparatuses. Accordingly, further power conservation has also been required for power supplies of electronic apparatuses. An example of power supplies of electronic apparatuses is a switching type power supply (hereinafter, called “switching power supply”) that drives a switching element, such as an FET (field-effect transistor), at a prescribed frequency and outputs a desired voltage. Among switching power supplies, there is a certain switching power supply that reduces the number of switching operations of the switching element to improve efficiency if a load is low in a normal operation (also called “normal mode”) state. Standards for power conservation have been annually revised. Not only in the normal operation state, but also in another state, further power conservation even in comparison with a low load operation has been required to further increase efficiency. Accordingly, a switching power supply outputs, for instance, DC 24 V as an output voltage in the normal mode. However, in a power saving mode for facilitating power conservation, the output voltage is, for instance, DC 3.2 V, which reduces standby power consumption.
Most of loss (power loss) in the switching power supply is caused by switching operations of the switching element that controls the output voltage of a transformer. Accordingly, reduction in the number of switching operations greatly contributes to improvement in efficiency of the switching power supply. Thus, it is often configured such that a time period of an on state of the switching element is extended to thereby increase energy of one switching operation, and intervals of the switching operations are increased accordingly to thereby reduce the number of switchings per unit time period.
For instance, Japanese Patent Application Laid-Open No. 2011-10397 discloses a power supply device for further reducing power consumption in a power conservation operation state. According to this power supply device, in a power conservation operation state where the output voltage of a transformer is set to a low voltage, control is performed according to which a switching element is turned off in response to a voltage induced in an auxiliary winding having a winding direction identical to a winding direction of a primary winding of the transformer, and the switching element is turned on based on the output voltage of the transformer.
Unfortunately, the switching power supply has the following problems. It is known that the transformer vibrates owing to exciting current flowing in an on state of the switching element to cause vibration noise. If the frequency of the vibration noise due to exciting current, that is, the switching frequency due to the switching element is in the human audible frequency band (in general, 20 Hz to 20 KHz), the vibration noise of the transformer offends human ears.
The switching frequency of the switching element is defined such that the switching frequency in a normal mode in a normal load state is fnml, the switching frequency in the normal mode in a low load state is fstb, and the switching frequency in a power saving mode is fpsv. In this case, the three switching frequencies generally have the following magnitude relationship.fnml>fstb>fpsv 
The switching frequency fstb in the normal mode in the low load state, and the switching frequency fpsv in the power saving mode are often in the human audible frequency band because the switching cycle of the switching element is long.
Furthermore, the higher the peak value of the exciting current (i.e., the drain current of FET as the switching element) flowing through the transformer is, the larger the vibration noise of the transformer is, which offends human ears. The peak value of the exciting current flowing through the transformer is defined such that the peak value of the exciting current in the normal mode in the normal load state is Inml, the peak value of the exciting current in the normal mode with the low load is Istb, and the peak value of the exciting current in the power saving mode is Ipsv. In this case, the peak values of the three exciting currents have the following magnitude relationship.Inml>Istb>Ipsv 
Thus, in the normal mode in the normal load state, the peak value of the exciting current Inml is high but the switching frequency fnml is out of the audible frequency band. Accordingly, the vibration noise of the transformer is inaudible to humans. In the normal mode in the low load state, the peak value of the exciting current Istb is high to a certain extent, and the switching frequency fstb is in the audible frequency band. Accordingly, the vibration noise of the transformer is large. On the contrary, in the power saving mode, the switching frequency fpsv is in the audible frequency band but the peak value of the exciting current Ipsv is low. Accordingly, the vibration noise of the transformer is small. Thus, the switching power supply has a problem in that the vibration noise of the transformer in the normal mode in the low load state is large.
To solve this problem, a switching power supply has been invented that is configured such that, in the normal mode and the power saving mode, the switching element always operates at the switching frequencies out of the audible frequency band (in general, 20 KHz or higher). However, such a switching power supply has a problem in that the power loss due to switching of the switching element in the power saving mode is large, which increases standby power consumption of the power supply device.
The switching frequency of the switching element is defined such that the switching frequency in the switching power supply in the high load state is fhigh, the switching frequency in the middle load state is fmid, and the switching frequency in the low load state is flow. In this case, the three switching frequencies generally have the following magnitude relationship.fhigh>fmid>flow
The switching frequency fmid in the middle load state, and the switching frequency flow in the low load state are often in the human audible frequency band because the switching cycle of the switching element is long.
The higher the peak value of the exciting current (i.e., the drain current of FET as the switching element) flowing through the transformer is, the larger the vibration noise of the transformer is, which offends human ears. The peak value of the exciting current flowing through the transformer is defined such that the peak value of the exciting current of the switching power supply in the high load state is Ihigh, the peak value of the exciting current in the middle load state is Imid, and the peak value of the exciting current in the low load state is Ilow. In this case, the peak values of the three exciting currents generally have the following magnitude relationship.Ihigh>Imid>Ilow
Thus, in the high load state, the peak value of the exciting current Ihigh is high but the switching frequency fhigh is out of the audible frequency band. Accordingly, the vibration noise of the transformer is inaudible to humans. In the middle load state, the peak value of the exciting current Imid is high to some extent, and the switching frequency fmid is in the audible frequency band. Accordingly, the vibration noise of the transformer is large. However, in the low load state, the switching frequency flow is in the audible frequency band but the peak value of the exciting current Ilow is low. Accordingly, the vibration noise of the transformer is small. Thus, the switching power supply has a problem in that the vibration noise of the transformer in the middle load state is large.
To solve this problem, a switching power supply has been invented that is configured such that, in any of the load states (the high load, the middle load and the low load), the switching element always operates at a switching frequency out of the audible frequency band (in general, 20 KHz or higher). However, such a switching power supply has a problem in that the power loss due to switching of the switching element in the low load state is large, which increases standby power consumption of the power supply device.
The present invention is made in such situations, and can reduce start-up power consumption while suppressing the vibration noise of the transformer.